1. Field of the Invention
The present invention relates, generally, to a high frequency signal transmission line having reduced noise. More particularly, the present invention relates to a signal transmission line having reduced radiation noise and reflection noise, by separating a conventional ground guard fence line disposed between signal lines into a plurality of ground line blocks that are spaced apart from each other to shield against noise.
2. Description of the Related Art
In general, high frequency signal transmission and high-speed switching of chips of electronic systems cause undesired noise due to electromagnetic interference. Particularly, a printed circuit board (PCB) for high frequency packages, which must have a high density circuit design, requires a predetermined wire length, and also, has wires transmitting a high frequency signal and wires transmitting a low frequency signal designed to be adjacent to each other. Thus, it is difficult to transmit the signal due to crosstalk noise generated by the two kinds of wires.
Noise generated upon signal transmission through microstrips of the PCB includes, for example, crosstalk noise, reflection noise, and radiation noise.
Crosstalk noise is generated by capacitive coupling and inductive coupling between two transmission lines constituting signal lines, and the magnitude thereof depends on the methods of designing transmission lines, insulating media and degree of impedance matching.
Also, crosstalk noise is divided into near-end crosstalk noise and far-end crosstalk noise.
Near-end crosstalk noise causes noise on the line adjacent to the high frequency line at the same time as a signal is transmitted from the signal-applied portion of the high frequency line to the far end thereof, and occurs when a signal is returned from the far end of the high frequency line to the signal-applied portion thereof.
Far-end crosstalk noise occurs when the signal is transmitted to the far end of the line. Although far-end crosstalk noise has a much smaller signal width than near-end crosstalk noise, it has a large magnitude, thus causing incorrect switching operation.
Further, reflection noise is generated upon mismatching of characteristic impedance or terminating resistance between the high frequency signal transmission line and the line adjacent to the transmission line. Reflection noise generated at the far end of the transmission line affects the near-end crosstalk noise. Hence, it is important to minimize not only reflection noise but also crosstalk noise that results from reflection noise.
Further, radiation noise is generated by a closed loop while the signal is transmitted along the closed loop that is formed in the signal transmission line.
In a conventional high density wire design to minimize crosstalk noise, a method of disposing a ground guard fence line (GGFL) between a high frequency signal line and a line adjacent to the signal line, and a structure having via holes formed through the GGFL have been used.
FIG. 1 is a perspective view showing the structure of a conventional high density wire design, composed of signal lines 11 and GGFLs 12 disposed between the signal lines 11 to minimize noise. FIG. 2 is a sectional view taken along the line A-A′ of FIG. 1, showing the GGFL 12, the via holes 13 and ground electrodes 14a and 14b. 
The via holes 13 are formed at regular intervals through the GGFL 12, and the ground electrodes 14a and 14b formed on upper and lower surfaces, respectively, of the GGFL 12 are connected to each other by means of the via holes 13.
As in FIG. 1, the conventional structure, which includes the signal lines 11, the GGFLs 12 disposed between the signal lines 11, and the via holes 13 formed at regular intervals through the GGFL 12, is designed so that the emission energy radiated from the signal line 11 is shielded by the GGFL 12.
However, all of the conventional structure shown in FIGS. 1 and 2, as well as other conventional structures, consider nothing but crosstalk noise, and do not consider reflection noise.
Thus, although the conventional structures are effective for reducing crosstalk noise, they have almost no effect of reducing reflection noise on a substrate having a high wire density. In this way, the conventional structures are disadvantageous because their simultaneous reduction of reflection noise and crosstalk noise is minimal.
In the conventional structure shown in FIG. 2, the returning signal is generated through the ground electrodes 14a and 14b positioned on the upper and lower surfaces of the GGFL 12. It is assumed that the high frequency signal is transmitted from the left to the right as shown by the arrow 16 and the returning signal is transmitted from the right to the left as shown by the arrow 17.
In this case, closed loops as shown by the dotted line arrows 18a and 18b are formed along the ground electrodes 14a and 14b positioned on the upper and lower surfaces of the GGFL 12 and the via holes connecting the upper and lower ground electrodes 14a and 14b, and hence, induces radiation noise.
As for reflection noise, the reason why it occurs is that capacitance C is caused due to the capacitive coupling between the ground line and the signal line, resulting in the characteristic impedance mismatching. To decrease reflection noise, the ground line should be designed to be spaced apart from the signal line by a predetermined interval. However, the structure thus designed does not exhibit the effect of a ground line to shield against crosstalk noise. That is, when the wire density is increased, reflection noise is increased again by the ground line that shields against crosstalk noise.
As for the transmission signal returning line formed on the GGFL 12, if the closed loop is formed by the GGFL 12, the ground electrodes 14a and 14b and the via holes 13, it induces radiation noise. However, in the conventional GGFL structure, when the signal lines are disposed to be adjacent to each other to shield against crosstalk noise, radiation noise is further strengthened on the GGFL 12, thereby decreasing the reliability of signal transmission.
With an increase in wire density, reflection noise, crosstalk noise and radiation noise should be considered at the same time. Among the three kinds of noise, if priority is given only to any one kind of noise, the other kinds of noise are inevitably generated. Thus, it is difficult to design an appropriate wire pattern.
In this regard, attempts to dispose GGFLs between signal transmission lines as a microstrip line on a PCB to remove crosstalk noise have been made, and are reported in “Effect Of Ground Guard Fence With Via And Ground Slot On Radiated Emission In Multi-Layer Digital Printed Circuit Board”, IEEE International Symposium On Electromagnetic Compatibility, 2001, pp. 653-656. Although the above method may reduce crosstalk noise, reflection noise and radiation noise still remain, or increase, thus worsening the overall noise properties of the PCB. It is impossible to solve the noise problems of PCBs using the above method.